Lightweight components for a consistently true-to-size mass production with high stiffness and structural strength are required for many areas of application. In vehicle construction, in particular, because of the saving in weight required, there is a high requirement for lightweight components made of thin-walled structures that nevertheless possess sufficient stiffness and structural strength. One method of achieving high stiffness and structural strength, despite the lowest possible weight of the component, calls for hollow parts that are made of relatively thin plate or plastics panels. Thin-walled plates, however, are prone to slight deformation. It has therefore already been known for some time, in the case of cavity structures, to foam out said cavity with a structural foam, whereby on the one hand the deformation or distortion is prevented or minimised, and on the other the strength and stiffness of said parts is enhanced. As regards two-dimensional parts of car bodies such as doors, roof parts, engine bonnets or boot lids, it is also known to enhance the stiffness and strength of said parts by layered laminates based on expandable or non-expandable epoxy resins or polyurethane resins being applied to said parts and connected firmly to the latter.
Conventionally, either such foamed reinforcing and stiffening means are metal foams or they contain a thermally curable resin or binder such as epoxy resins. Said compositions contain as a rule a propellant, fillers and reinforcing fillers such as hollow microbeads made of glass. Preferably such foams have in the foamed-up and cured state a density of 0.3 to 0.7 g/cm3. Said foams are to withstand without harm, for at least a short period of time, temperatures of more than 150° C., preferably more than 180° C. Such foamable, thermally curable compositions contain as a rule further constituents such as curing agents, process aids, stabilizers, dyes or pigments, optionally UV absorbers and adhesion-enhancing constituents.
When structural components of car bodies are stiffened with expandable thermally curable resin compositions, said resins are very often cured during the passage of the car bodies through the paint drying ovens. The curing temperatures applied in the painting process are relatively low and the temperature acting on the car body may be very irregularly distributed, so that the body exhibits colder parts during the passage through the painting oven, which in the case of some thermally expandable foams prevents a full expansion. In addition, the temperature profile within a painting oven is irregularly distributed (for example: the temperature in the bottom area may be significantly lower than in the roof area of the oven). This results in thermally curing foam parts in the bottom area of the vehicle body being exposed to relatively low curing temperatures. This means that thermally curable foamable compositions for the above-mentioned areas of application also have to exhibit a high degree of expansion at relatively low temperatures.
EP-A-0 798 062 proposes components of metallic foam material in which the metallic foam material is made from a metal powder and propellant and is optionally formed between solid metal plate parts in a press at high temperatures and high pressures. Such a process is suitable only for large-sized components that are produced separately outside the assembly line of a motor vehicle and then incorporated in the standard assembly process. The insertion and foaming up of metallic foam materials is not possible under the process conditions of a standard motor vehicle assembly line.
U.S. Pat. No. 4,978,562 describes a specifically light, reinforcing door bar of a composite material consisting of a metal tube which is partly filled with a specifically light polymer of cellular structure. It is proposed to mix curing resins based on epoxy resins, vinyl ester resins, unsaturated polyester resins and polyurethane resins with the corresponding curing agents, fillers and cell-forming agents in an extruder, to cure said mixture to a core and to insert it into the metal tube in such a way that the core is fixed in the tube by friction forces or mechanically. Alternatively the polymer core may be manufactured of liquid or pasty polymer material by casting and be pressed into the tube. Reactive, heat curable and thermally expandable mouldings are not disclosed.
U.S. Pat. No. 4,769,391 describes a preformed composite insertion part for insertion into a hollow structural member. Said insertion part contains a multiplicity of thermoplastic granules consisting of a mixture of a thermoplastic resin and non-expanded, expandable hollow microbeads and a matrix of expanded polystyrene, which holds the aforementioned granules. The thermoplastic resin of the granules may be a thermoplastic, such as a thermoplastic polyester for example, or it may be a heat curable epoxy resin. After the insertion of the part into the hollow body to be filled the component is heated to a temperature which produces a “vaporising” of the expanded polystyrene—vaporising means here the breakdown of the expanded polystyrene to a thin film or carbon black. At the same time the thermoplastic granules expand and cure in some cases, wherein cavities of greater or lesser size remain between the individual expanded granule particles as a function of the degree of expansion of the granules.
In a similar manner U.S. Pat. No. 4,861,097 and U.S. Pat. No. 4,901,500 describe specifically lightweight composite bars of foamed polymers and metallic structures for the reinforcing of vehicle doors. According to this teaching the polymer core part is first of all formed by the production of a liquid or pasty reinforcing material which is then injected or cast into a channel-type structure and then cured. Thereafter, said cured core part is inserted into the hollow metallic structure. Alternatively, the core may be preformed or precast by injection moulding and then inserted into the cavity.
WO 89/08678 describes a process and compositions for the reinforcing of structural elements in which the polymer reinforcing material is a two-component epoxy system in which the one component is a paste-type material based on epoxy resins and the second component is a mixture of fillers, a pigment and a liquid curing agent of pasty consistency. Directly prior to the charging of the reinforcing material into the hollow structure the two components are mixed, inserted into the hollow structure and cured, wherein the hollow structure may optionally be pre-heated.
WO 96/37400 describes a W-shaped reinforcing structure which contains a thermally expandable, resin-type material and prior to the curing is inserted into the hollow member to be reinforced. The reinforcing polymer matrix consists preferably of a single-component, paste-type system containing an epoxy resin, an acrylonitrile-butadiene rubber, fillers, high-strength glass beads, a curing agent as well as an accelerator and a propellant based on an azo compound or a hydrazide compound.
WO 98/15594 describes foamed products for applications in the automobile industry based on preferably liquid, two-component epoxy systems in which the one component consists of a liquid epoxy resin and metal carbonates or bicarbonates and the other component consists of pigments, optionally hollow microbeads and phosphoric acid. During the mixing of the two components said compositions cure with foaming up. Applications for the reinforcement or stiffening of hollow structures are not disclosed.
The polymeric materials of the aforementioned prior art are either not suitable for the production of preformed moulded parts which expand thermally at a later stage through heating and are also heat curable, or, if they are suitable, they have as a rule a highly tacky surface which leads to contamination of the bearing surfaces, and conversely binds dirt and dust. In addition, a tacky surface of said moulded parts prevents the handling and in particular the storage, e.g., the stacking, of several parts on top of one another. For this reason moulded parts of the state of the art are provided with a protective film which is removed immediately prior to use. Such protective films make the production and application of such moulded parts more expensive, however, particularly as the protective film has to be disposed of after removal, which causes additional costs.
In order to reduce the surface tack of such moulded parts, WO00/52086 proposes making heat curable, thermally expandable mouldings from a mixture consisting of at least one solid reactive resin, at least one liquid reactive resin and at least one reactive resin with a flexibilising effect, together with curing agents and/or accelerators or propellants. Said mouldings are suitable for the stiffening and/or reinforcement of thin-walled metal structures and for the stiffening of hollow lightweight metal constructions. Compared with known heat curable, thermally expandable mouldings, the mouldings are characterised according to the teaching of said document by improved dimensional stability in the uncured state and by low surface tack. The properties of processability and dimensional stability are achieved by the mixing of epoxy resins with different melting points. However, the reduced surface tack, for example, is always to be achieved only in a very narrowly defined temperature range, so that a formulation which is admittedly tack-free in the winter exhibits a highly tacky surface in the summer. Furthermore, said procedure calls for the use of large amounts of expensive resins and curing systems. Particularly as regards the low-cost production of such expandable mouldings by the injection moulding method, manufacturing and handling difficulties continually arise. This is undesirable in terms of the reliability of the manufacturing process.
U.S. Pat. No. 4,444,818 describes a thermally curable adhesive laminate which is composed of a heat curable resin layer in the form of a “prepreg” in which a reinforcing material is embedded. Said document further proposes attaching a flat-pressed tubular material to one side of the prepreg, which may assume its original tubular form again on the heating of the reinforcing laminate. The prepreg laminate may consist of two different, thermally curable resin layers. Epoxy resins are proposed as binders for the thermally curable layers of the prepreg. The tubular or hose-type member is to consist of polyethylene, ethylene-vinyl acetate copolymers, polypropylene, polystyrene or PVC or else nitrile rubber. The method of manufacture for such reinforcing laminates is cumbersome.
EP-A-230 666 describes a process for producing a single-component heat curable composition which on heating forms a Urethane-Epoxy-Silicon Interpenetrating Network (IPN) system. Said document proposes producing from said compositions metal-reinforcing laminates (“patches”) which adhere directly to oil-containing metal surfaces such as oily steel plates. The IPN is to be formed by a polyepoxy compound, a blocked polyamine curing agent and a polyurethane prepolymer with extended chain in which some isocyanate groups of the prepolymer are blocked with a hydroxyfunctional polysiloxane.
EP-A-297 036 describes a laminate consisting of a support, e.g., resin-bonded glass fibre tissue, to which a layer of heat curable resin is applied. In order to protect the tacky resin surface, a covering foil of a material shrinking under the action of heat is provided. Said film is to be provided with slits which expand to an open position after a thermal pre-treatment, so that a part of the tacky surface is exposed. It is therefore no longer to be necessary to withdraw the protective film prior to the application of the laminate. No details are given on the composition of the tacky resin layer.
EP-A-376 880 describes a laminate arrangement for the stiffening of two-dimensional bodies incorporating a supporting layer of a curable synthetic resin material in which a reinforcing material connected to the latter or embedded in it is provided. In addition, an adhesive layer applied to the supporting layer and facing the member to be reinforced is provided, which incorporates a curable synthetic resin material provided optionally with fillers and other additives. In order to achieve as high a reinforcing effect as possible without deformation of the two-dimensional member (plate), the adhesive layer is to possess after the curing of the synthetic resin a higher modulus of elasticity than the cured synthetic resin material of the supporting layer, and at the same time supporting layer and adhesive layer are to exhibit in the cured state at least approximately the same coefficient of thermal expansion as the two-dimensional member to be stiffened. The supporting layer is to consist of a glass fibre tissue and a mixture of liquid epoxy resins and solid epoxy resins together with curing agents, the adhesive layer is to consist substantially of heat curable, self-adhesive synthetic resins, which are likewise made up of liquid and solid epoxy resins together with curing agents and fillers.
In a similar manner EP-A-298 024 describes a process for stiffening plates and plastics mouldings with the aid of a single- or multilayered two-dimensional stiffening member, in which at least one layer consists of a synthetic resin curable under the effect of heat. Said stiffening member is to be subjected first of all to a first thermal treatment in which at least one surface of the stiffening member becomes tacky as a result of said first thermal treatment. After this the stiffening member is to be applied with the tacky surface to the element to be stiffened and after this the stiffening member is to be subjected to a second thermal treatment until all the layers of the stiffening member are cured. It is proposed that one layer of the reinforcing member is composed of heat curable epoxy resins, which optionally contains glass fibre tissue. There Is proposed as a second layer, which is to become tacky during the first thermal treatment, a hot melt adhesive on an epoxy base, in certain cases on a polyurethane or copolyester base. Alternatively said layer is to consist of a film shrinking under the effect of heat, so that a tacky layer is exposed after shrinkage.
WO 95/27000 describes a curable, injectable composition for the reinforcing of thin, rigid plates or panels. The composition is made up of heat curable resins, expandable hollow microbeads and particulate reinforcing material of ground glass fibres, ground carbon fibres and their mixtures. The various epoxy resins based on glycidyl ethers, glycidyl esters and glycidyl amines are proposed as heat curable resin compositions.
CA-A-2 241 073 describes a film-reinforcing stiffening laminate for rigid, thin-walled substrates. According to the teaching of said document the polymer is to cure in a paint oven with expansion and at the same time bond intimately with the inner surface of the base substrate to be reinforced. Details of the binder composition are not given in said document.
It follows from the documents cited above that substantially epoxy-based binder compositions or compositions based on polyurethanes are proposed for surface- or frame-stiffening laminates. Although as a rule the latter provide the required degree of stiffening, they do not meet the requirement for a chemical base which is acceptable in industrial hygiene and health terms. Reactive polyurethane systems still contain in almost all cases residues of monomeric diisocyanate. For this reason the workplaces where such compositions are used have to be fitted with suitable suction equipment, in order to be able to protect the persons employed in said workplaces against exposure to isocyanates. With epoxy-based systems on the one hand the dimensional stability is determined by the composition of the epoxy resin mixture, in which case an attempt is made to prevent or at least minimize the portion of liquid epoxy resins with a molecular weight of below 700, since said low molecular weight epoxy compounds may trigger allergic or sensitising reactions on skin contact. On the other hand, such uncured laminates or mouldings with a high proportion of liquid, low molecular weight epoxides exhibit a good adhesion to the substrates to be stiffened, but are less resistant to process fluids such as washing and cleaning baths, phoshatizing and conversion baths, and to electrophoretic paint. In particular the washing fluids are applied at high pressure and temperatures up to 75° C.
In order to obtain a high structural strength and in particular compressive strength in the case of mouldings with low specific weight, many of the above-mentioned documents propose the use of hollow glass microbeads as a lightweight filler, which is nevertheless to ensure sufficiently high compressive strength of the cured foam material. Said hollow glass microbeads are also a major component in reducing the weight and in achieving a high compressive strength, since in this way a controlled collapse of the structure of the foam during the compression test or during an accident involving a collision is brought about. The use of hollow glass microbeads nevertheless has some serious disadvantages:                They are expensive and thus increase the cost of the structural foam, since they have to be used on a considerable scale.        Surface-active substances such as silanes or titanates, which, as is known, are used to improve the adhesive properties of the structural foam on the metal substrates, react with the surface of the hollow glass microbeads and hence reduce the effect of the latter at the interface with the substrate. The adhesion properties of the structural foam are thereby impaired considerably, in particular after heat or the build-up of moisture.        The process for manufacturing the structural foams is complicated by the fact that the brittle hollow glass microbeads are pressure- and shear-sensitive, so that compositions that contain hollow glass microbeads may be pumped only at relatively low pressures and also may be extruded or worked by injection moulding only at relatively low pressures. This leads to long process times and hence high production costs. In addition, the destruction of some of the hollow microbeads cannot be entirely excluded, whereby the density of the structural foam decreases in an undesirable manner.        
The inventors therefore set themselves the object of replacing the hollow glass microbeads with “neutral” fillers, with which the collision and compressive strength properties of the thermally expanded and cured foam compositions are to remain, and at the same time the efficient use of silanes and titanates and similar compounds for optimizing the adhesion properties is to be possible. In the compression test the desired fracture behaviour is, in a similar way to foams containing the hollow glass microbeads, to be distinguished by the fact that the structural foam does not experience brittle failure under load, but instead the structure is noticeably destroyed with progressive loading and as high a force level as possible may be maintained.